The toner density sensor is the most important component in an image forming device for providing optimal image quality. The toner density sensor includes a light emitting element that radiates light, a light receiving element, and an amplifier. The light receiving element receives the light radiating from the light-emitting element and reflected from the detection object; the amplifier increases the amplitude of the detection voltage from the light receiving element.
The image forming device may be configured to form images using an intermediate transfer process where a toner image formed on an intermediate transfer belt through primary transfer is then formed on paper through secondary transfer. In this case, the toner density sensor irradiates the intermediate transfer belt with light from the light emitting element, and uses the light receiving element to detect the light reflected by the toner image on the intermediate transfer belt. The light receiving element generates a photoelectric current in response to the received light intensity. The image forming device detects the density of the toner on the intermediate transfer belt by detecting the voltage from the photoelectric current generated. Consequently, the image forming device then makes optical or electrical adjustments on the basis of the detection results.
Note that the light emitting element and the light receiving element may be mounted on the surface of a substrate (printed substrate) in the toner density sensor, and the sensor may be configured so that the optical path from the light emitting element runs parallel to the substrate; this created a problem where light radiating in directions other than a prescribed direction along the optical path creates optical noise. This optical noise may also be called stray light; stray light reduces the detection accuracy.
That is, light radiating from the light emitting element mounted to the surface of a substrate contains desirable components that travel toward the detection object. However, this light also contains components traveling in other directions, such as components traveling towards another device on the substrate, or components traveling toward the substrate where a portion thereof enters substrate. When light components other than the desirable components traveling towards the detection object arrives at the light receiving element, the non-desirable components change the value of the detection voltage, making highly accurate detection less likely to take place.
The optical noise, which causes the detection accuracy to degrade, comes from components that travel into the substrate and reach the light receiving element, and components that travel through the air outside the substrate and reach the light receiving element. The optical noise traveling outside the substrate can be considered separately as the optical noise traveling through the air in the plane of the substrate, and the optical noise traveling through the air outside the substrate in a plan view of the substrate.
Phenolic resin (applied to paper) and glass-epoxy resin and the like are typically used for substrates. These resins provide comparatively good optical transmittance. Therefore, light entering the substrate will reflect inside the substrate while a portion of the light radiates outside the substrate from the area surrounding the light receiving element that has no copper foil, thereby reaching the light receiving element.
Japanese Patent No. 4531357 attempts to minimize this kind of optical noise that travels inside the substrate. More specifically, Japanese Patent No. 4531357 proposes providing a slotted through-hole between the light emitting element and the light receiving element which are mounted on the surface of the substrate. With this configuration, the optical noise traveling inside the substrate radiates from the through-hole. Consequently, the configuration reduces the amount of optical noise reaching the light receiving element. Japanese Patent No. 4531357 also proposes providing a light shielding component on a case that covers the light emitting element and the light receiving element. The light shielding element inserts into the through-hole.